1. Field of the Invention
This invention relates to a disk recording and/or reproducing apparatus, and is best applicable to the apparatus used for hard disks and having an optical encoder for controlling positions of a head with respect to the disks.
2. Description of the Prior Art
U.S. Pat. Nos. 4593194, 4625109, 4647769, etc. are prior arts of the optical encoder for regulating the head always into its correct position when the apparatus is used for the hard disk. All the head arms included in the optical encoders shown in such prior arts are constituted as. illustrated in FIG. 18.
In FIG. 18, a pair of upper and lower floating heads 204 used for recording information on and/or reproducing information from the hard disk 203 are secured to one end of an aluminium head arm 202 which is swung on an arm shaft 201, and a stainless-steel wing 205 is secured to the other end of the head arm 202. Further, secured to the free end of the wing 205 is a glass scale plate 207 of the optical encoder 206.
On the glass scale plate 207, scale marks 208 extending in the radius direction of the arm shaft 201 are formed into an arc round the arm shaft 201, and on the upper side and the lower side of the scale plate 207, a light-emitting element 210 and a light-sensitive element 211 of a photo coupler 209 are fixedly arranged, respectively, so that the photo coupler 209 can detect the scale marks.
When the hard disk 203 is rotated at high speed on the one hand, and the head arm 202 is swung, on the other hand, in a direction of arrow a on the arm shaft 201 by a motor (not shown), so as to record information on or reproduce information from the hard disk 203 by the use of the head 204, the glass scale plate 207 is moved together with the head arm 202, and thereby, tracking by the head 204 is controlled due to optical detection of the scale marks 208 by the photo coupler 209.
However, in the optical encoder disclosed in the prior art, thermal offtrack errors are often produced. As . shown in FIG. 18, when the center PC of the photo coupler 209 coincides with the central line SC of the scale marks 208, the head 204 is positioned over the central line MD of the recorded zone of the hard disk 203, which is defined by an outer limit OD and an inner limit ID. At that time, if the temperature rises, the aluminum head arm 202 and stainless-steel wing 205 expand in a direction of axis Y, and as the result, the glass scale plate 207 is moved in a position indicated in two-dots-and-dash lines in FIG. 18. But the coefficient of expansion of glass is much smaller than those of aluminium and stainless steel, so that point A.sub.1 on the glass scale plate 207 moves to point A.sub.2, because the glass scale plate 207 slightly expands in a direction of axis X, as compared with the expansion of the head arm 202 and wing 205 in the direction of axis Y.
If the head arm 202 is moved on the same number of tracks under the different atmospheric temperatures, the different angles of rotation .theta..sub.1 and .theta..sub.2 are made. In other words, there is produced the thermal offtrack error 2 corresponding to the difference of the angles (.theta..sub.1 -.theta..sub.2). A larger coefficient of glass must be selected in order to move point A.sub.1 to point A.sub.3 shown in FIG. 18.
Moreover, in the prior art, the center of gravity of the assembly which consists of the head arm 202, wing 205 and glass scale plate 207 is positioned apart from the center of the arm shaft 201, so that it is difficult to move the head 204 at high speed. In addition, it is very difficult to assemble the wing 205 and the scale plate 207, because the center of the scale marks 208 must be made to coincide with the center of the arm shaft 201 with accuracy of about 10 .mu.m, when the scale plate 207 is adhered to the wing 205.